The following links are from the Special Metals website, producer of Inconel alloys:

http://www.specialmetals.com/publication/tech_bulletin_welding.pdf

http://www.specialmetals.com/publication/tech_bulletin_625.pdf

"Normal" procedures, for "normal" situations, this joint would normally be welded with NiCrMo-3 (Inconel 625) However, per most codes you would be required to qualify your own procedure if it is to be welded according to any type of industry standard.  Also, many situations that seem to be "normal" turn out not to be, so be on your guard.

Hi Sandra,
I think that you need to post a bit more info.  Since you say that the pipe is clad with 625.  What sort of thickness is this?  what is inside pipe?  Is overlay due to corrosive enviroment outside pipe?  What is external enviroment? and finally what sort of pressure is inside carrier pipe?  If you want a take-off that will be rated to the carrier pipe you may be forced into putting a carbon steel sweep-o-let into the pipe and then overlaying the exterior with welded overlay to give you corrossion resistance.  If you put the W-O-L onto the 625 cladding you are relying on the cladding to steel bond, which may/may not be OK.